function p=periods(iorder)
%PERIODS tidal periods, names, speeds, etc.
% PERIODS returns a structure of tidal periods, names, speeds,
% strength ratios, and descriptions.  The order of the constituents
% can be returned based on descending relative strengths, increasing
% period, or grouped by species type (diurnal, semi-diurnal,...).
% The user can append additional constituents to the structure. For
% example, to add M4 to the bottom of the structure:
%    p(20).name={'M4'};
%    p(20).hours=12.42/2;
%    p(20).ratio=[];
%    p(20).speed=[];
%    p(20).desc={'First M2 Harmonic'};
%
% Call as: p=periods(iorder);
%
% The structure looks like:
%   p =
%
%  1x19 struct array with fields:
%      name
%      hours
%      ratio
%      speed
%      desc
%
% To extract just the hours, for example to send to the
% harmonic analysis routine HARMONIC, use
%    per=[p.hours];
%
% Input: iorder - return order indicator (optional)
%                 0 = grouped by species type (default)
%                 1 = descending relative strengths
%                 2 = increasing period (decreasing speed)
%
% The following constituents are included.
%
%     Period  in     Standard    Relative  Speed in    Description
%     solar hours      Name      Strength  Deg/Sol hr
%   ---------------------------------------------------------------------
%  Semidiurnal
%       12.42	        M2	  1.00     28.98410    Principal Lunar
%       12.00	        S2	  .466     30.00000    Principal Solar
%       12.66	        N2	  .192     28.43973    Larger Lunar Elliptic
%       11.97	        K2	  .127     30.08214    Lunisolar Semidiurnal
%       12.01	        T2	  .027     29.95893    Larger Solar Elliptic
%       12.19	        L2	  .028     29.52848    Smaller Lunar Elliptic
%       12.91	        2N2	  .025     27.89535    Lunar Elliptic Second Order
%       12.63	        NU2	  .036     28.51258    Larger Lunar Evectional
%       12.22	        Lambda3   .007     29.45563    Smaller Lunar Evectional
%       12.87	        MU2	  .031     27.96821    Variational
%  Diurnal
%       23.93	        K1	  .584     15.04107    Lunisolar Diurnal
%       25.82	        O1	  .415     13.94304    Principal Lunar Diurnal
%       24.07	        P1	  .194     14.95893    Principal Solar Diurnal
%       26.87	        Q1	  .079     13.39866    Larger Lunar Elliptic
%       24.84	        M1	  .033     14.49205    Smaller Lunar Elliptic
%       23.10	        J1	  .033     15.58544    Small Lunar Elliptic
% Long-period
%      327.67           Mf	  .172     1.09803     Lunar Fortnightly
%      661.30           Mm	  .091     0.54437     Lunar Monthly
%     4383.00           Ssa	  .080     0.08214     Solar Semiannual
%

if nargin==0
   iorder=0;
else
   if iorder<0 |  iorder>3
      error('iorder to PERIODS out of range')
   end
end

p(1).name={'M2'};
p(1).hours=12.42;
p(1).ratio=1.000;
p(1).speed =28.98410;
p(1).desc ={'Principal Lunar'};

p(2).name={'S2'};
p(2).hours=12.00;
p(2).ratio=.466;
p(2).speed =30.00000;
p(2).desc ={'Principal Solar'};

p(3).name={'N2'};
p(3).hours=12.66;
p(3).ratio=.192;
p(3).speed =28.43973;
p(3).desc ={'Larger Lunar Elliptic'};

p(4).name={'K2'};
p(4).hours=11.97;
p(4).ratio=.127;
p(4).speed =30.08214;
p(4).desc ={'Lunisolar Semidiurnal'};

p(5).name={'T2'};
p(5).hours=12.01;
p(5).ratio=.027;
p(5).speed =29.95893;
p(5).desc ={'Larger Solar Elliptic'};

p(6).name={'L2'};
p(6).hours=12.19;
p(6).ratio=.028;
p(6).speed =29.52848;
p(6).desc ={'Smaller Lunar Elliptic'};

p(7).name={'2N2'};
p(7).hours=12.91;
p(7).ratio=.025;
p(7).speed =27.89535;
p(7).desc ={'Lunar Elliptic Second Order'};

p(8).name={'NU2'};
p(8).hours=12.63;
p(8).ratio=.036;
p(8).speed =28.51258;
p(8).desc ={'Larger Lunar Evectional'};

p(9).name={'Lambda3'};
p(9).hours=12.22;
p(9).ratio=.007;
p(9).speed =29.45563;
p(9).desc ={'Smaller Lunar Evectional'};

p(10).name={'MU2'};
p(10).hours=12.87;
p(10).ratio=.031;
p(10).speed=27.96821;
p(10).desc={'Variational'};

p(11).name={'K1'};
p(11).hours=23.93;
p(11).ratio=.584;
p(11).speed=15.04107;
p(11).desc={'Lunisolar Diurnal'};

p(12).name={'O1'};
p(12).hours=25.82;
p(12).ratio=.415;
p(12).speed=13.94304;
p(12).desc={'Principal Lunar Diurnal'};

p(13).name={'P1'};
p(13).hours=24.07;
p(13).ratio=.194;
p(13).speed=14.95893;
p(13).desc={'Principal Solar Diurnal'};

p(14).name={'Q1'};
p(14).hours=26.87;
p(14).ratio=.079;
p(14).speed=13.39866;
p(14).desc={'Larger Lunar Elliptic'};

p(15).name={'M1'};
p(15).hours=24.84;
p(15).ratio=.033;
p(15).speed=14.49205;
p(15).desc={'Smaller Lunar Elliptic'};

p(16).name={'J1'};
p(16).hours=23.10;
p(16).ratio=.033;
p(16).speed=15.58544;
p(16).desc={'Small Lunar Elliptic'};

p(17).name={'Mf'};
p(17).hours=327.67;
p(17).ratio=.172;
p(17).speed= 1.09803;
p(17).desc={'Lunar Fortnightly'};

p(18).name={'Mm'};
p(18).hours=661.30;
p(18).ratio=.091;
p(18).speed= 0.54437;
p(18).desc={'Lunar Monthly'};

p(19).name={'Ssa'};
p(19).hours=4383.00;
p(19).ratio=.080;
p(19).speed= 0.08214;
p(19).desc={'Solar Semiannual'};

if iorder==0
   iperm=[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19];
elseif iorder==1   %  ordering for decreasing ratio
   iperm=[1 11 2 12 13 3 17 4 18 19 14 8 15 16 10 6 5 7 9];
elseif iorder==2   %  ordering for increasing period
   iperm=[4 2 5 6 9 1 8 3 10 7 16 11 13 15 12 14 17 18 19];
end

p=p(iperm);
%
%        Brian O. Blanton
%        Spring  1999
%